Method of making a capillary for electrokinetic transport of materials

ABSTRACT

A method for fabricating a capillary element for electrokinetic transport of materials. The method comprises providing a first capillary element which has a first capillary channel disposed through its length. The capillary channel comprises first and second ends and an outer surface. A continuous layer of an electrically conductive material is applied along a length of the outer surface such that the continuous layer of electrically conductive material extends along the outer surface to a point proximal to, but not up to at least one of the first and second ends. The capillary element is then segmented into at least first and second separate capillary element portions at an intermediate point of the capillary element and the continuous layer.

Microfluidic technology has been heralded as a significant technologicaladvance in a number of areas, including biological research, clinicaldiagnostics, environmental monitoring, pharmaceutical screening, and avariety of others. The advantages associated with this technology aremyriad and compelling. For example, the use of small material volumes,digitally controlled fluidics, and sensitive chemistries and detectionschemes allows rapid, automatable, reproducible and accurate analyticalmethods in the above-described areas.

Unfortunately, some of the benefits of microfluidic technology can bedifficult to realize. For example, microfluidic systems require onlyvery small amounts of material to perform a given analysis, e.g., in thepicoliter to nanoliter range. However, conventional fluid handlingtechnologies, e.g., pipettors, pumps, dispensers and the like, typicallyare not capable of operating at such small volumes, generally operatingabove the microliter range. As a result, any advantages of reducedvolumes are generally lost in introducing fluids into the microfluidicsystems, because larger amounts are dispensed into reservoirs of thedevice.

One particularly useful method of introducing extremely small volumes ofmaterials into the microfluidic devices is described in U.S. Pat. No.5,779,868, which describes a pipettor capillary that is integrated withthe channels of the microfluidic device. Materials are introduced intothe channels of the device by sipping them through the capillaryelement. Using this improvement, one can readily sample nanoliter andeven picoliter volumes of materials into the microfluidic system,thereby realizing this promise of microfluidics.

The present invention generally provides improved devices and methods offabricating microfluidic systems having such a capillary element.

SUMMARY OF THE INVENTION

The present invention generally provides methods of fabricatingmicrofluidic devices that include an external pipettor element having anintegrated electrical contact/electrode. The advantages of the presentinvention are that the electrode is disposed up to the open terminus ofthe capillary element through a simple fabrication process.

In particular, provided is a method for fabricating a capillary elementfor electrokinetic transport of materials. The method comprisesproviding a first capillary element which has a first capillary channeldisposed through its length. The capillary channel comprises first andsecond ends and an outer surface. A continuous layer of an electricallyconductive material is applied along a length of the outer surface suchthat the continuous layer of electrically conductive material extendsalong the outer surface to a point proximal to, but not up to at leastone of the first and second ends. The capillary element is thensegmented into at least first and second separate capillary elementportions at an intermediate point of the capillary element and thecontinuous layer. As a result, the first portion of the capillaryelement comprises the first end and a first intermediate end, and thesecond portion comprises the second end and a second intermediate end.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a top (Panel A) side (panel B) and perspective (PanelC) view of a high-throughput microfluidic analytical device.

FIG. 2 schematically illustrates the fabrication of a pipettor elementin accordance with the present invention.

FIG. 3 schematically illustrates the pipettor element shown in FIG. 2 inconjunction with a microfluidic channel network, which functions as ahigh-throughput microfluidic analytical device.

FIG. 4 (Panels A and B) schematically illustrate an alternateconfiguration of the high-throughput microfluidic devices of the presentinvention.

FIG. 5 schematically illustrates the interaction of capillary elementsas described herein with fluids which are being drawn into thecapillary.

DETAILED DESCRIPTION OF THE INVENTION

The present invention generally provides improved microfluidic devicesand methods of fabricating microfluidic devices and systems that areuseful in high throughput analysis of sample materials. In particular,the present invention is directed to improvements in the fabrication ofcapillary elements that are joined with the channel containing portionof the devices.

Background of Pipettor devices

As noted above, introduction of samples into microfluidic channelnetworks has generally involved sacrificing one of the many promises ofthis otherwise promising technology. Specifically, while microfluidicsystems typically are able to process fluid samples in the nanoliterrange, conventional fluid handling technologies are generally limited tothe microliter range, which ends up being the smallest effective volumeof a sample fluid that one can use. A solution to this problem wasprovided in U.S. Pat. No. 5,779,868, which describes a microfluidicdevice having a body structure incorporating an integrated channelnetwork for carrying out a variety of different analyses. A pipettor orcapillary element is provided having a channel disposed through it suchthat the capillary channel is in fluid communication with at least onechannel in the channel network that is disposed in the body structure.The capillary element may be a separate capillary element that isattached to the body structure, or it may be an extension of the bodystructure, e.g., fabricated from the same substrate as the bodystructure. Examples of alternative structures of such pipettorelement/body structures are described in Published International PatentApplication No. WO 98/00705, which is incorporated herein by reference,in its entirety for all purposes.

In one particular embodiment, electrokinetic forces are used to drawfluids or other materials into the capillary element by either or bothof electrophoresis and/or electroosmosis, and then transport thatmaterial into the channel network wherein an appropriate analysis iscarried cut. Application of the electric fields necessary for suchelectrokinetic transport requires that an electrical connection be madeboth within the channel network, and within the reservoir of materialthat is to be drawn into the capillary channel and channel network.Particular methods and systems for making this latter connection includethe provision of a metallic layer on the outside surface of thecapillary element, e.g., by sputtering metal or applying a metallicpaint on that surface, and connecting an appropriate electrical lead tothat layer to deliver an appropriate voltage or current to that layer,and thus to the sample reservoir it contacts. Alternatively, a simplewire or wires are provided adjacent to or coiled around the capillaryelement, which function as the electrode or electrodes that contact thesample material reservoir.

An example of a previously described device is illustrated in FIG. 1.Specifically, FIG. 1 is a schematic illustration of a microfluidicdevice and integrated pipettor element from a top (Panel A), side (PanelB) and perspective view (Panel C). As shown, the device 100 includes amain body structure 102 that includes a channel network disposed in itsinterior. The channel network includes a main analysis channel 104,which fluidly connects a sample inlet 106 with waste reservoir 108. Tworeagent reservoirs 110 and 112 are provided in fluid communication withthe analysis channel 104 via channels 114 and 116, respectively. Reagentreservoirs 110 and 112 are paired with buffer/diluent reservoirs 118 and120, respectively, which are in communication with channels 114 and 116via channels 122 and 124, respectively. In order to prevent electrolyticdegradation of reagent and/or buffer materials, each of reservoirs 108,110, 112, 116 and 120 is provided in electrical and/or fluidcommunication with an electrical access reservoir/salt bridge channel128a/b, 130a/b, 132a/b, 134a/b, and 136a/b, respectively. The provisionof an electrical access reservoir/salt bridge allows the application ofvoltages via electrodes for long periods of time without resulting insubstantial degradation of reagents, buffers or the like. It should benoted that as reservoir 108 is a waste well, it typically does notrequire a separate electrical access reservoir/salt bridge, e.g.,128a/b.

The device also includes a capillary element 138 which includes aninternal capillary channel running its length, the capillary channelcommunicating with the analysis channel 104 via the sample inlet 106.Although shown as being perpendicular to the main body structure of thedevice 102, it will be appreciated that the capillary element can becoplanar with the body structure, e.g., extending in the same plane asthe body structure and collinear with the analysis channel, e.g., asdescribed in Published International Application No. WO 98/00705, whichis incorporated herein by reference. Panel B of FIG. 1 also illustratesan electrical power supply 140 which provides a potential gradientbetween a sample reservoir into which the pipettor element is insertedand the channel network of the device, by applying different voltagelevels to electrode 142, which contacts the sample fluid reservoir,e.g., a well on a multiwell plate, and electrode 144 which contactsfluid within the channel and/or reservoir network of the device.

While the methods of integrating electrodes into the high-throughputdevices have proven effective for providing the appropriate electricalconnection, often such electrodes are imperfect, and do not extend tothe end of the capillary element. This can give rise to a number ofpotential problems. Initially, when an electrode doesn't extend the fulllength of the capillary, an adequate electrical connection with thefluid reservoir requires inserting the capillary element further intothe reservoir.

A more typical problem is that upon removal of the capillary andelectrode from the sample material, flow of material through thecapillary element into the channel network is stopped, because theelectrical circuit is broken. Thus, in high throughput systems wheremultiple different samples are drawn serially into the capillary channeland channel network, e.g., from different wells in a multiwell plate,the flow of material is intermittently started and stopped. Further, inorder to ensure that all materials are subject to the same conditions,other materials flowing through the remainder of the channel networkmust be started and stopped. In addition to the problems this causes forthe assay biochemistry, it also represents a substantial time waste, inthat the time spent transitioning between sample reservoirs iseffectively lost, i.e., nothing is being analyzed.

Devices fabricated according to the presently described methods,however, remedy many of these problems. In particular, in accordancewith the fabrication methods described herein a first capillary elementis provided which includes the capillary channel disposed through itslength. As noted previously, this capillary element may be separate orintegral to the body structure of the channel network of the device.

A continuous layer of an electrically conductive material is appliedalong a length of the outer surface of the capillary element. By"continuous layer" is meant that a layer of material is continuous alongthe length of the capillary element. The layer may or may not becontinuous around the circumference or perimeter of the capillaryelement, e.g., it can be a uniform coating of the capillary element orit can be a simple stripe applied down the length of the capillary.

The electrically conductive layer is typically applied to a point thatis proximal to at least, and in some cases, both ends of the capillaryelement. By "proximal to" is meant that the conductive layer typicallyextends to a point that is within about 5 mm, typically within about 2mm, preferably within about 1 mm of the end of the capillary element,more preferably within about 0.5 mm and often within about 0.2 mm oreven 0.1 mm of the end of the capillary.

Generally, a small amount of open space is left at the end of thecapillary element, in order to permit insertion of the capillary elementinto the body structure of the overall device, if appropriate to thechosen fabrication method. Depending upon the method used, the uncoatedends may be protected from coating by masking off the ends of the outersurface of the capillary element using a protective layer, e.g., tape orother covering.

Typically, the conductive layer may be applied by any of a variety ofmethods that are well known in the art for metalizing surfaces. Forexample, a metallic layer may be sputtered onto the outer surface of thecapillary. Alternatively, a thin metallic sheet or foil may be appliedaround the capillary element, which, in turn may be thermally oradhesively bonded to the outer surface. Whether sputtered, wrapped orotherwise, typically preferred metallic layers are generally selectedfrom those metals that are widely used in electronics applications,including, e.g., platinum, chrome, titanium, tungsten, and rhodium. In afurther alternative method, a metallic or electrically conductive paintis simply painted onto the outer surface of the capillary element.

The capillary element is then cut or otherwise segmented at anintermediate point along the length of the capillary and within thecontinuous layer. Typically, this segmenting is carried out by any of anumber of means, including, e.g., sawing, scoring and breaking, or thelike. By cutting or segmenting the capillary at an intermediate point inthe conductive layer, a new end is created, which has the conductivelayer extending up to that end. This end is then employed as thesampling end of the ultimate capillary element. In particular, in thecase of a separate capillary element that is attached to the bodystructure, the original end with the conductive layer extending up to apoint proximal to the end, is the end that is attached to the bodystructure. This is typically accomplished by inserting the capillaryelement into an aperture in the body structure that is configured toreceive the capillary.

This process is schematically illustrated in FIG. 2, where an initialcapillary element 200 having a capillary channel 202 disposed through itis provided with a continuous layer 204 of conductive material depositedupon its outer surface 208. Although shown as a cylindrical capillary,it will be appreciated that any capillary shape can be used inaccordance with the methods described herein, including rectangular,polygonal, e.g., octagonal, etc., elliptical, and amorphous. The layerof conductive material extends substantially the entire length of thecapillary, but not the entire length. In particular, outer surfaceportions 208a and 208b are left uncoated by the conductive layer. By notcoating the capillary element 200 up to the end, one reduces the chancesof fouling the open end of the capillary with the coating. The capillaryelement 200 is segmented at intermediate point 206, which as shown,produces two capillary portions 200a and 200b each having anintermediate end 206a and 206b, respectively, which has the conductivelayer 204 extending up to it.

Attachment of the capillary element to the body structure, if such isthe structure of the device, is generally carried out by providing ahole or aperture that approximates the size of the capillary element,e.g., the outer circumference or perimeter, such that the capillary canbe inserted into the hole. Typically, the hole or aperture isdimensioned to receive the capillary element, and thus is slightlylarger than he capillary element. Thus, for circular capillaries, thishole or aperture may be provided by drilling into the body structure ofthe device using, e.g., an ultrasonic or high-speed drill. The hole ispositioned such that when the capillary is inserted into it, thecapillary channel is in fluid communication with at least one of thechannels within the body structure. FIG. 3 schematically illustrates thejunction of a microfluidic device 300 containing a channel network 302with a capillary element 200a having an outer coating 204 which extendsup to the end 206a of the capillary, as described with reference to FIG.2. As shown, the uncoated end 202a of the capillary element 204a isinserted into a hole 306 that is disposed in the body structure 308 ofthe device 300, such that the channel 202 of the capillary element 200ais in fluid communication with the channel network 302. As noted above,fabrication of the hole or aperture into which the capillary element isinserted, may be accomplished by a number of means. Typically, forexample, for cylindrical capillaries, this aperture is drilled into thesubstrate or body structure at an appropriate point, e.g., to providefor connection between the capillary element and the channel network.Alternative microfabrication methods are also useful, including, e.g.,etching the aperture or fabricating the body structure, e.g., byinjection molding, embossing, stamping or other methods, to include theaperture.

FIG. 3 also schematically illustrates an electrical controller attachedto conductive layer 204 via electrical lead 312, and in electricalcommunication with fluid in the reservoirs or channels of channelnetwork 302 via electrode/electrical lead 314.

Alternatively, rectangular capillary elements may be used. In suchcases, the hole or aperture may be provided by etching a square orrectangular aperture into the body structure such that the capillary isappropriately positioned. Such rectangular capillary elements aredescribed in, e.g., U.S. Patent Application Ser. No. 09/173,469, filedOct. 14, 1998, which is incorporated herein by reference for allpurposes.

An example of a device similar to that shown in FIG. 1, but including acollinear, substantially rectangular capillary element, is shown in FIG.4A. The same reference numerals are used for elements that are commonbetween FIGS. 1 and 4. As shown, the overall device 100 again includes amain body structure 102 as described with reference to FIG. 1, whichincludes integrated channel network disposed in its interior. Therectangular capillary element 438 includes a capillary channel 440running its length. The capillary element is attached to the bodystructure via a rectangular opening 442 in the body structure 102.Insertion of a rectangular end of the capillary element 438 intorectangular opening 442 places the capillary channel 440 into fluidcommunication with at least one of the channels in the integratedchannel network within the body structure.

Because the opening 442 in the body structure is substantiallyrectangular, it is more conveniently fabricated than circular openings.In particular, while circular openings are typically drilled or airabraded into a body structure, rectangular openings are moreconveniently fabricated by fabricating rectangular notches in twosubstrates by, e.g., photolithographic methods, which are mated todefine the body structure of the device. The two notches are positionedto provide a single rectangular opening in the body structure. FIG. 4Billustrates an expanded view of the joining of a rectangular capillarywith a two-layer microfluidic device. As shown, the device comprises atwo-layer body structure including the above-described notches. Asshown, the body structure 102 is made up of at least first and secondplanar substrates 402a and 402b, respectively. The upper surface of thelower substrate 402a includes grooves fabricated therein, whichcorrespond to the desired channel structure of the finished device,e.g., groove 404. The upper substrate 402b is mated and bonded to theupper surface of the lower substrate 402a (as illustrated by the dashedarrows). Typically, bonding is carried out by thermal bondingtechniques, which result in a single integrated unit having sealedchannels or conduits running through its interior. The upper substratealso typically includes a number of holes disposed through it (notshown), which holes align with and provide access to the channels of thefinished device. The lower and upper substrates also include notches442a and 442b, respectively, which are aligned when the two substratesare mated, to define an opening. Although these notches could be of anyshape, e.g., rectangular, hemispherical, trapezoidal, etc., it isgenerally easier to fabricate substantially rectangular notches, e.g.,using the same fabrication techniques and steps used in fabricating thegrooves/channels of the device 100, e.g., groove 404. Substantiallyrectangular notches produce a substantially rectangular opening alongthe edge of the body structure of the device. The notches generallyrange in depth depending upon the dimensions of the rectangularcapillary element to be inserted therein. Typically, however, thesenotches will range in depth from about 10 μm to about 50 μm, and will befabricated to make the transition from the channel in the capillaryelement to the channel in the device's body structure. For example,where a capillary element has a wall thickness of 15 μm (e.g., minoraxis or interior diameter of 15 μm, with wall thickness of 15 μmyielding overall cross section of 45 μm), the notch 442a on the lowersubstrate 402a will typically be approximately 30 μm deep, e.g.,allowing for 15 μm wall thickness and a 15 μm deep channel which matchesup with the minor axis of the capillary element, while the notch 442b onthe upper substrate 402b will be approximately 15 μm deep to accommodatethe upper wall of the capillary element. The notches typically extendinto the substrate, e.g., away from the edge, up to about 2 mm, in orderto conveniently and fixedly receive the capillary element.

A substantially rectangular capillary element 438 is then inserted andattached to the body structure 402 via the opening (as shown by thedashed arrow). Typically, attachment of the capillary element isaccomplished using an adhesive, e.g., epoxy, although other bondingtechniques may also be used depending upon the nature of the materialsused, e.g., thermal bonding, solvent welding, etc.

Although the capillary element 438 is shown as being collinear with themain analysis channel 404 of the device 100, it will be readily apparentthat the rectangular capillary element can be curved or bent out of theplane of the channel network to provide a more useful samplingcapillary. Bent capillaries can be held in the bent shape, e.g., byapplying a rigid bent sheath, i.e., plastic sheath or a coated sheath ofpolyimide or Teflon (polytetrafluoroethylene) or the like, over thecapillary element to hold the capillary in the bent or curvedorientation. Alternatively, a rectangular capillary can extend out ofthe plane of the channel network, e.g., perpendicular to the channelnetwork plane, e.g., as shown in FIG. 1. In particular, rectangularopenings could be readily fabricated into the lower substrate 402a usingwell known fabrication techniques, e.g., etching.

In a further alternative method, the capillary element may comprisemerely an extension of the body structure of the device itself, throughwhich a channel has been provided. The fabrication of such a capillaryelement is generally carried out as described in published InternationalPatent Application No. 98/00705, which is incorporated herein byreference. In the case of devices fabricated in this way, the methods ofthe present invention are still practical. Specifically, the conductivelayer is provided over the outer surface of the capillary elementportion of the device, extending up to a point proximal to the open endof the capillary portion. The capillary portion is then cut or choppedat an intermediate point in the conductive layer, creating a new end ofthe capillary element, wherein the conductive layer extends up to thenew end of the capillary.

Following assembly of the overall device, an electrical lead that iseither coupled to or connectable to an electrical power source, isconnected to the conductive layer on the capillary element, in order todeliver an appropriate voltage or current through that layer. Connectingthe electrical lead is generally carried out by methods known in theart, e.g., soldering, or adhesively attaching the lead.

As noted above, the devices fabricated according to the methodsdescribed herein provide a number of advantages. For example, byproviding the conductive layer extending to the sampling end of thecapillary element, one is able to maintain an electrical circuit throughthe capillary channel for a greater amount of time. In particular, theelectrical circuit is established substantially from the moment thecapillary element contacts the fluid it is sampling, without dipping thecapillary deeper into the well. This permits sampling the material witha smaller likelihood of contamination from previously sampled materials,as well as faster sampling by not requiring.

In addition, the conductive layer permits the maintaining of theelectrical circuit even after the capillary element is withdrawn fromthe sampled fluid. Specifically, once the capillary element is withdrawnfrom the sample fluid, a drop remains on the tip of the capillaryelement, e.g., as shown in FIG. 5 and with reference to FIGS. 2 and 3.As the drop of fluid 502 extends across the surface of capillary end206a of the capillary element 500, it will maintain the electricalcircuit between the conductive layer 204 and the fluid in the capillarychannel 504. By maintaining the electrical circuit, even while thecapillary is out of the sample or other fluid reservoir, one canmaintain flow of material through the capillary channel, as well as theremainder of the channel network. As noted above, this can amount to asubstantial time savings in high throughput systems which requirefrequent movement of the capillary element from one fluid reservoir toanother.

As an example, assuming that an assay employs a set of spacer fluidsbetween each sample plug that includes a low salt space with high saltguard bands on each side of the sample plug, e.g., as described in WO98/00705, previously incorporated herein each sampling cycle requiresthat the capillary element make four well to well shifts, during whichtime the assay operation would typically be suspended. If each shiftrequires one second, then four seconds are lost during each cycle. For a96 well plate, this amounts to nearly six and a half minutes. Assumingthat one is screening a modest library of 10,000 different compounds,the time waste amounts to approximately eleven hours. By maintaining theelectrical circuit intact during these transition periods, as ispossible using the methods described herein, one can recover these timelosses.

Although generally described with reference to the fabrication ofcapillary element containing microfluidic devices, it will beappreciated that the methods described herein have use in any instancewhere one wishes to provide an integrated electrical connection to thevery end of a capillary element, e.g., as a ring electrode, or the like.

All publications and patent applications are herein incorporated byreference to the same extent as if each individual publication or patentapplication was specifically and individually indicated to beincorporated by reference. Although the present invention has beendescribed in some detail by way of illustration and example for purposesof clarity and understanding, it will be apparent that certain changesand modifications may be practiced within the scope of the appendedclaims.

What is claimed is:
 1. A method for fabricating a capillary element forelectrokinetic transport of materials, comprising:providing a firstcapillary element having disposed through its length, a first capillarychannel, the capillary channel comprising first and second ends and anouter surface; applying a continuous layer of an electrically conductivematerial along a length of the outer surface, the continuous layer ofelectrically conductive material extending along the outer surface to apoint proximal to, but not up to at least one of the first and secondends; segmenting the capillary element into at least first and secondseparate capillary element portions at an intermediate point of thecapillary element and the continuous layer, the first portion comprisingthe first end and a first intermediate end, and the second portioncomprising the second end and a second intermediate end.
 2. The methodof claim 1, wherein the applying step comprises applying a conductivemetal layer to the capillary element.
 3. The method of claim 2, whereinthe conductive metal layer is sputtered onto the outer surface of thecapillary element.
 4. The method of claim 2, wherein the conductivemetal layer comprises a metal selected from platinum, chrome, titanium,tungsten, and rhodium.
 5. The method of claim 1, wherein the conductivelayer comprises a metallic paint that is painted onto the outer surfaceof the capillary element.
 6. The method of claim 1, wherein thesegmenting step comprises sawing the capillary element into the firstand second portions at the intermediate point.
 7. The method of claim 1,further comprising the step of attaching at least one of the first andsecond ends of the first and second portions of the capillary element toa substrate having at least a first channel disposed in an interiorportion thereof, such that the capillary channel extending the length ofthe first or second capillary portion is in fluid communication with theat least first channel disposed in the substrate.
 8. The method of claim7, wherein the attaching step comprises providing the substrate with acavity disposed therein, the cavity being dimensioned for receiving thefirst or second end of the capillary element.
 9. The method of claim 8,wherein the cavity is fabricated by drilling a hole into at least onesurface of the substrate, the hole providing fluid communication withthe first channel in the interior portion of the substrate.
 10. Themethod of claim 8 wherein the cavity comprises an etched opening in atleast one surface of the substrate, the etched opening providing fluidcommunication with the first channel in the interior portion of thesubstrate.
 11. The method of claim 8, wherein the capillary elementprovided in the providing step comprises a rectangular cross-sectionalshape, and the cavity comprises a rectangular shape dimensioned toreceive the capillary element.
 12. The method of claim 1, furthercomprising attaching an electrical lead to the continuous layer of thefirst or second portion.
 13. The method of claim 1, wherein thecontinuous layer is applied to a point that is greater than 0.1 mm fromat least one of the first and second ends.
 14. The method of claim 1,wherein the continuous layer is applied to a point that is greater than0.2 mm from at least one of the first and second ends.
 15. The method ofclaim 1, wherein the continuous layer is applied to a point that isgreater than 0.5 mm from at least one of the first and second ends. 16.The method of claim 1, wherein the continuous layer is applied to apoint that is greater than 1 mm from at least one of the first andsecond ends.
 17. The method of claim 1, wherein the continuous layer isapplied to a point that is greater than 2 mm from at least one of thefirst and second ends.